1. Field of the Invention
This invention relates generally to trigger generators, and in particular to a trigger generator for use with radar equipment which senses and compensates for inherent time delays through the radar equipment, and provides a trigger pulse and variable delay pretrigger pulse for firing the radar transmitter and driving peripheral equipment.
2. Description of the Prior Art
In conventional radar equipment a periodically recurring trigger pulse is applied to the radar modulator causing the transmitter to fire a burst of radio frequency energy through the radar antenna. The conventional modulator may incorporate pulse forming network coupling coils, and damping networks, all of which contribute to an inherent time delay between triggering the modulator and firing the output RF burst. Adding to this time delay, the transmitter, which may comprise a magnetron or other transmitting tube, requires a finite time in which to draw current after the normal voltage is applied to it. Such inherent time delays are not constant, but vary with environmental conditions such as temperature and humidity and operational condition.
In some conventional radar equipment, a pretrigger is used to indicate that the radar transmitter is about to fire. For instance, although not normally required in commerical marine radars, a marine vessel traffic system (VTS) may require a pretrigger to drive peripheral equipment. A vessel traffic system might be constructed in which the transmitter and antenna are at a remote location with respect to the master station. With such an arrangement the radar trigger pulse might be generated at the remote location with certain timing and operator control signals being transmitted via a microwave or hard wire communication link from the master station. A pretrigger pulse might also be generated to enable or turn on peripheral equipment in advance of the transmitter being fired. The peripheral equipment might be located at the master station or at some other remote location. Frequently, it is desirable that the timing of this pretrigger should be adjustable with respect to the firing of the radar transmitter. However, due to the inherent time delays through the radar modulator and transmitter, it has not heretofore been possible to accurately adjust the timing of the pretrigger without a prior knowledge of the factors and characteristics of the radar in its environment which contribute to this time delay. It will be appreciated that these conditions, such as temperature and humidity, may differ between the master station and the remote site.
The prior art technique for providing a pretrigger has been to place a monostable multivibrator between the radar trigger generator and the radar modulator. A monostable multivibrator receives the radar trigger pulse and generates a delayed pulse using a resistor capacitor (RC) network to achieve the desired time delay. The radar trigger is then treated as a pretrigger and the output of the monostable multivibrator is applied to the radar transmitter as a trigger pulse. The time delay between the pretrigger and trigger pulse is adjustable by means of the RC time constant. It will be seen that this method does not account for the inherent time delay through the radar modulator and transmitter. Thus the pretrigger timing must be based upon an assumed nominal time delay through the radar modulator and transmitter. Another shortcoming of this technique is that the timing period of the monostable multivibrator changes as a function of supply voltage, temperature, and component aging. Thus often sophisticated power supply regulating techniques, close thermal tolerances, and frequent routine maintenance are required to assure system performance.